Privacy is not all-or-nothing, like a switch. Apple has been playing a cat-and-mouse game with ad companies in which they provide a token for advertisers in exchange for disallowing fingerprinting, and then ad companies abuse that, so they provide a different rotating token and do more to disallow fingerprinting, and then ad companies abuse that, so they clamp down even more, and then ad companies abuse something else, so now they're clamping down even more, and presumably ad companies will figure out still more ways to abuse something, and so on.
There is no such thing as a third-party guarantee that doesn't also disallow useful functionality.
I get that I'm being hard on Apple. And I understand that there's no way to guarantee third-party apps.
But I still maintain that they should have explicitly warned users that installing third-party apps would put their privacy at risk. The situation with Android is undoubtedly far worse. But Google doesn't (at least, not plausibly) claim to be privacy-friendly.
I mean, consider what lengths Apple has gone to to keep customers from rooting their phones. Back in the day, they were bricking them. And they justified it all for security against malicious apps.
There is no such thing as a third-party guarantee that doesn't also disallow useful functionality.